1. Field of Invention
This invention relates to imaging systems. Specifically, the present invention relates to millimeter wave electromagnetic imaging systems.
2. Description of the Related Art
Electromagnetic imaging systems are employed in a variety of demanding applications including security surveillance, aircraft landing, missile guidance, target detection and classification, terrain mapping, and mine detection applications. Such applications require cost-effective, space-efficient, and reliable imaging systems that can provide effective images at night and in adverse weather conditions.
Imaging systems are either passive or active. Both passive and active electromagnetic imaging systems are typically named in accordance with the frequency band of electromagnetic energy employed by the systems.
Active electromagnetic imaging systems, such as radar systems, transmit electromagnetic energy toward a scene and detect the reflected electromagnetic energy to generate an image of the scene. Unfortunately, in warfare applications, an enemy may detect the transmitted electromagnetic energy, making the systems undesirable in many applications.
Passive electromagnetic imaging systems generally lack transmitters. Passive systems detect electromagnetic energy emanating from a scene and convert the energy into electronic signals. Consequently, passive imaging systems are generally immune to enemy detection.
Optical and infrared passive imaging systems are currently in wide spread use. Such systems, however, typically require favorable weather conditions to produce effective mission-enabling images. Clouds, rain, or snow may block optical and infrared electromagnetic energy. Millimeter wave electromagnetic energy however, passes through clouds, rain, and snow with little attenuation. Consequently, millimeter wave imaging systems are generally less sensitive to weather obstructions than their optical and infrared counterparts.
Millimeter wave systems typically employ a focal plane array (FPA) of electromagnetic energy detectors. The detectors convert received millimeter wave electromagnetic energy into electronic signals that are fed to millimeter wave super heterodyne receivers and accompanying microelectronic circuits. The super heterodyne receivers and accompanying microelectronic circuits are expensive, bulky, consume excess power, and typically offer relatively poor resolution.
Electromagnetic diffraction creates detector size versus wavelength design limitations that may further increase system size, especially in systems employing full 256xc3x97256 detector FPAs. FPA detectors, also termed FPA pixels, are sized in accordance with wavelength of the electromagnetic energy detected by the system. Typically, the pixels must not be significantly smaller than the wavelength of the detected electromagnetic energy.
In many applications, the present state of the art results in very large systems. Space-constraints necessitate the use of a partial FPA and an accompanying mechanical scanning mechanism. Typically, the scanning mechanism scans the focal plane, taking pixel measurements. The pixel measurements are multiplexed onto a single channel and subsequently provided to image processing circuits such as pixel integration circuits. The mechanical scanning mechanisms may further increase system size and cost while reducing system reliability and performance. In addition, the systems exhibit a trade-off between signal-to-noise ratio (SNR) and image integration time. Image integration time is the time spent correcting signals from each pixel in the FPA. To achieve effective SNRs, the systems may require undesirably long image integration times. Therefore, systems employing mechanical FPA scanners typically cannot operate in real-time and have compromised SNRs. Consequently, conventional millimeter wave imaging systems are impractical in many applications.
Hence, a need exists in the art for a cost-effective, space-efficient, and power-efficient staring electromagnetic imaging system that can produce relatively high-resolution real-time images day or night and in virtually all weather conditions.
The present invention addresses the need for a low cost, low power consumption, small, lightweight, high performance, millimeter wave imaging system. In the illustrative embodiment, the inventive system adapted for use millimeter wave electromagnetic energy and includes a first mechanism for receiving electromagnetic energy of a first wavelength from a scene and providing electromagnetic energy of a second wavelength in response thereto. The second wavelength is shorter than the first wavelength. A second mechanism measures variations of the electromagnetic energy of the second wavelength over a predetermined area and provides the information about the scene in response thereto.
In a specific embodiment, the system is a millimeter wave imaging system and the first wavelength and the second wavelength are on the order magnitude of millimeters. The variations result from differences of emissivity and/or reflectivity coefficients of features of the scene. The second mechanism includes a section of material having a resistivity that varies in accordance with variations in temperature. A low-frequency video read-out circuit measures changes in the resistivity of the section of material in response to changes in electromagnetic energy received by the section of material. An array of the sections of material, which are bolometers, is thermally isolated from a surrounding environment. The bolometers are included in a full frame bolometer focal plane array.
In a more specific embodiment, the first mechanism includes a lens having an index of refraction greater than 1. The lens is transparent to electromagnetic energy of millimeter wavelengths but generally opaque to infrared electromagnetic energy. The array of bolometers is positioned relative to the lens so that the electromagnetic energy of the second wavelength impinges on the array of bolometers and is absorbed by the same. The array of bolometers is positioned parallel to an output aperture of the lens and within distance d less than  less than xcex of the output aperture, where xcex is the second wavelength. The operation frequency of the system is between approximately 10 GHz and 10 THz.
In an alternative embodiment, an array of patch antennas receives the electromagnetic energy of the second wavelength and feeds the electromagnetic energy of the second wavelength through apertures in a ground circuit and to a microstrip feed circuit. The microstrip feed circuit includes a first line and a second line along which a first portion of the electromagnetic energy of the second wavelength and a second portion of the electromagnetic energy of the second wavelength propagate, respectively. The first line is longer than the second line by a factor of xcex/2, where xcex is the second wavelength. A bolometer is connected at a first end to the first line of the microstrip feed circuit and is connected at a second end to the second line of the microstrip feed circuit. An array of the microstrip feed circuits feeds a corresponding array of bolometers. The bolometers of the array are mounted on a substrate and connected to the microstrip feed circuits. The bolometers are enclosed in a vacuum. A low-frequency line-by-line video read-out circuit senses the temperature change due to electromagnetic energy of the second wavelength transferred to the bolometers and provides parallel video signals in response thereto. The second mechanism further includes a mechanism for processing the parallel video signals and displaying an image corresponding to the scene in response thereto. The second mechanism also includes a mechanism for rejecting infrared electromagnetic energy and passing millimeter wave electromagnetic energy. The second mechanism further includes for rejecting includes an antireflectivity layer, and a thermoelectric cooler.
The novel design of the present invention is facilitated by the first mechanism, which reduces the wavelength of input electromagnetic energy. The wavelength reduction results in improved image resolution, enhanced signal-to-noise ratio, and improved image acquisition time. The mechanisms for incorporating an array of thin film bolometers as direct sensors for the millimeter wave radiation obviate the need for expensive super heterodyne receivers and associated microelectronics. These advantages permit use of a full staring focal plane array without accompanying focal plane array scanning mechanisms, which are often slow, expensive, and unreliable.